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Sequoia joins $120m series C in Robinhood CEO’s AI startup

Harmonic, an AI startup based in Palo Alto co-founded by Robinhood CEO Vlad Tenev, has raised US$120 million in a series C round led by Ribbit Capital at a US$1.5 billion post-money valuation.

The round also included participation from Sequoia Capital, Index Ventures, Kleiner Perkins, and new investor Emerson Collective.

Harmonic is developing AI models for mathematical reasoning and formal verification, with its flagship Aristotle model recently made available to the public.

The company said Aristotle achieved gold-medal-level performance at the International Mathematical Olympiad.

Aristotle now offers plain English input, automated lemma generation, and a streamlined terminal interface.

Harmonic previously secured US$100 million in series B funding in July 2025 and US$75 million in series A in September 2024.

Other backers include Paradigm and Era Funds.

🔗 Source: Harmonic

🧠 Food for thought

Implications, context, and why it matters.

  • Harmonic’s Aristotle reached gold-medal-level performance by solving five of six 2025 International Mathematical Olympiad problems with formal Lean 4 proofs, an interactive theorem prover and machine-verifiable proof language; it treats a problem as solved only when the proof checks in Lean 1.
  • Aristotle links a Lean proof search with an informal reasoning module, then adds a geometry solver built on Yuclid’s deductive database 1. The setup handles category theory (the study of structures and relationships between them) and homological algebra (tools from algebra used to study topological and algebraic structures) 1. It adds lemmas to Mathlib and catches textbook errors 1.
  • As models mature, teams will plug formal methods into CI/CD (Continuous Integration/Continuous Delivery) workflows 2. That opens space for DevOps and security vendors to ship integrations, managed proof infrastructure, plus compliance-grade logging. Certora, a provider of formal verification tools for smart contracts (self-executing blockchain programs), says its prover integrates with CI/CD 2. In a proposal for Compound (a decentralized lending protocol in Decentralized Finance, or DeFi), it outlined a VSCode (Visual Studio Code) extension for ongoing verification for smart contracts 2.
  • Popular CI/CD platforms like GitHub Actions, GitLab CI/CD, and Jenkins (developer automation services) could be integration points 3. Specialized providers could offer automated proof checking as a service, while DeFi protocols, where Certora says formal verification has helped prevent bugs and avert losses, look like near-term buyers 2.

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